Fire sprinkler systems are a well-known type of active fire suppression system. Sprinklers are installed in all types of buildings, commercial and residential, and are generally required by fire and building codes for buildings open to the public. Typical sprinkler systems comprise a network of pipes, usually located at ceiling level, that are connected to a reliable water source. Automatically actuated valves called sprinkler heads are disposed along the pipes at regular intervals. Each sprinkler head is operative to open automatically in the event of a fire. For example, one design of sprinkler head includes a fusible element, or a frangible glass bulb, that is heat-sensitive and designed to fail at a predetermined temperature. Failure of the fusible element or glass bulb opens the valve, allowing water to flow through the head, where it is directed by a deflector into a predetermined spray pattern. Sprinkler systems may suppress a fire, or inhibit its growth, thereby saving lives and limiting inventory loss and structural damage. Sprinkler specifications are published by the National Fire Protection Association (e.g., NFPA 13, 13D, 13R).
The sprinkler system (more generally, Fire Protection System, or FPS) is fed from a pump room or riser room. In a large building the FPS consist of several “zones,” each being fed from a riser in the pump room. The riser contains a main isolation valve and other monitoring equipment (e.g., flow switches, alarm sensors, and the like). The riser is typically a 6 or 8 inch diameter pipe coupled through a booster pump (called the fire pump) to the main water supply to the building. The riser then progressively branches off into smaller “cross mains” and branch lines, also known as “zones”. At the furthest point from the riser, typically at the end of each zone, there is an “inspector's test port,” which is used for flow testing. Numerous other valves, such as for filling and/or purging the pipes, testing internal pressure, measuring gas or water properties, and the like, may be included in the FPS pipes.
FPS may be of the “wet” or “dry” types. In a “wet” system the sprinkler pipes in each room are full of water under a predetermined “internal set point” pressure. If the water pressure decreases below the set point, valves are opened and/or a pump is activated, and water flows into the sprinkler pipes in an attempt to maintain the pressure. The set point pressure drops when water escapes the system, such as due to the opening of a sprinkler head in a fire.
To prevent damage to equipment or merchandise by water leaking from the FPS in conditions other than a fire, and in environment conditions in which water in the pipes may freeze, “dry” system are used. A dry FPS uses compressed air in the piping as a “supervisory gas.” The air is maintained at a supervisory pressure, e.g., typically ranging between 13-40 PSI. When a sprinkler head opens, the air pressure drops to atmospheric (e.g., 0 PSI), and a valve opens in response to the lower pressure. The valve locks in the open position and water rushes into the system. One type of dry FPS, known as a pre-action, provides increased protection against water damage by increasing the probability that the system is only activated by an actual fire. A pre-action FPS requires one (e.g., Single Interlock) or more (e.g., Double Interlock) action signals before water is injected into the system—for example, both a drop in supervisory air pressure and a signal from a heat or smoke detector.
Building codes specify a minimum angle, measured from the horizontal, at which wet FPS pipe is to be hung. The purpose of this angle is to ensure that water flows to the end of the pipe, so that the internal volume of the pipe is full of water along its entire length, minimizing the delay in water discharge when a sprinkler head opens. Also, codes specify that air vents can be installed at the far end of each pipe from the street valve, to purge air from the pipe interior as the system is “charged” (i.e., when water is initially introduced). However, in practice, there are usually one or more “high” or elevated points in the FPS wet pipe system where air is trapped. This air includes oxygen (O2), which reacts with the water and pipe steel to cause corrosion, which may be of either galvanic or organic origin. Sometimes, microbes can grow in the water and accelerate the corrosion by means of the byproducts produced during their metabolic cycle. This is called Microbiologically Influenced Corrosion (MIC). Over time, MIC or galvanic corrosion can cause extensive damage to a wet FPS, eventually resulting in leaks. Both the damage caused by leaking water, and the need to replace corroded FPS pipes, provide significant incentive to minimize or eliminate wet FPS corrosion due to O2 within the pipes.
One approach to solving this problem is to purge atmospheric air from the FPS pipes using an inert gas, such as nitrogen (N2), prior to charging the system. Nitrogen is an inert gas, and pure N2 contains no oxygen. However, commercially common means of generating N2, such as by membrane-filtering atmospheric air, generate N2 in the range of 95%-98% purity and Pressure Swing Adsorption systems generate N2 in the range of 95%-99.999% purity; accordingly, this N2 may contain some concentration of O2. Additionally, nitrogen has a dew point of −40° F., meaning it can absorb water vapor (as well as other gases dissolved in the water) at any higher temperature.
Water usually contains dissolved oxygen—that is, O2 molecules, apart from the oxygen bound up in the H2O molecules forming the water itself. As one example, a test of local city water at 60 degrees F. in Charlotte, N.C. revealed an O2 content of 9.617 ppm (parts per million). Due to the partial pressure of gases, O2 from such water will outgas into the pockets containing N2, providing enough O2 for the onset of detrimental corrosion. Accordingly, simply purging wet FPS pipes with N2 prior to charging the system is not a long-term solution to corrosion.
Deoxygenating water—the process of reducing the number of free oxygen molecules dissolved in water—prior to charging a wet FPS system is known. Water may be deoxygenated by exposure to low-O2-concentration gas and/or vacuum conditions to draw O2 and other residual free gasses out of the water, causing the dissolved O2 to “outgas” into the lower-concentration gas or vacuum. It is known to use N2 gas to deoxygenate water for wet FPS systems. For example, U.S. Patent Application Publication No. 2011/0226495 discloses a wet FPS system having a water reuse tank and in-line static mixer. The reuse tank is filled with sufficient fresh water to fill the FPS pipe volume. This water is circulated from the tank through the in-line static mixer, with N2 gas being injected in the circulation line from an N2 generator. The water is circulated through the in-line static mixer until a desired level of deoxygenation is achieved. As this reference discloses, such a system is effective to deoxygenate water to approximately 0.1 ppm (parts per million) of O2. When the FPS is drained for testing or maintenance, the deoxygenated water is retained in the water reuse tank, for reuse when the FPS is again made operational. Reusing the water avoids the need to spend the considerable time required to deoxygenate fresh water by circulation through the in-line static mixer.
The system disclosed in US 2011/0226495 presents numerous deficiencies. A water reuse tank of sufficient volume to hold all of the water in the wet FPS is required. Such a tank is large and heavy, requiring ample space and possibly structural reinforcement to deploy it in many buildings. The oxygenation process is lengthy, requiring continuous recirculation of water in the reuse tank through the in-line static mixer, in the presence of N2 gas, to achieve a sufficient level of deoxygenation. Additionally, separate piping systems for circulating water and gas to/from the reuse tank, each with multiple valves that must be coordinated and controlled, result in significant complexity and cost, increase maintenance requirements, and introduce potential failure points. Furthermore, such a system may ultimately limit the amount of deoxygenation that can be achieved, leaving some residual amount of O2 gas dissolved in the water, which may eventually outgas into the FPS piping.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.